It may be well here to explain to students that one of the lines of argument which lead to the conclusion that the water molecule, as it ordinarily exists, is really complex and massive, is based upon measurements of the Faraday dielectric constant for water; for this constant, or "specific inductive capacity," is found to be very large, something like 50 times that of air or free ether; whereas for glass it is only 5 or 6 times that of free space. The dielectric constant of a substance generally increases with the density or massiveness of its molecule,—indeed, the value of this constant is one of the methods whereby matter displays its interaction with and loading of the free ether of space,—and any such density as the conventional nine times that of hydrogen for the molecule of water would be wholly unable to explain its immense dielectric constant.
The influence of the massiveness of a water molecule is also displayed in its power of tearing asunder or dissociating any salts or other simple chemical substance introduced into it; common salt, for instance, is found always to have a certain percentage of its molecules knocked or torn asunder directly it is dissolved in water, so that, in addition to a number of salt molecules in solution, there are a few positively charged sodium atoms and a few negatively charged chlorine atoms, existing in a state of loose attraction to the water aggregate, and amenable to the smallest electric force; which, when applied, urges the chlorine one way and the sodium the other way, so that they can be removed at an electrode and their place supplied by freshly dissociated molecules of salt, thus bringing about its permanent electro-chemical decomposition, and enabling the water to behave as an electrolytic conductor directly a little salt or acid is dissolved in it.
The power of the water molecule to associate itself with molecules of other substances is illustrated by the well-known fact that water is an almost universal solvent. It is its residual affinity which enables it to enter into weak chemical combination with a large number of other substances, and thus to dissolve those substances. The dissolving power usually increases when the temperature is raised, possibly because the self-contained or self-sufficient groupings of the water molecules are then to some extent broken up and the fragments enabled to cling on to the foreign or introduced matter instead of only to each other. The foreign substance is apt to be extruded again when the liquid cools, and when the affinity of the water-aggregates for each other resumes its sway. Very hot water can dissolve not only the substances familiarly known to be soluble in water, but it can dissolve things like glass also; so that glass vessels are unable to retain water kept under high pressure at a very high temperature, approaching a red heat.
Another material which also seems to have the power of combining with a number of other bodies, under the influence of the loose mode of chemical combination spoken of as residual affinity, is carbon; so that a block of charcoal can absorb hundreds of times its own bulk of certain gases.
Indeed, Sir James Dewar has recently employed this absorbing power of very cold carbon to produce a perfect kind of vacuum, which may, perhaps, be the nearest approach to absolute vacuum that has yet been attained: probably higher than can be attained by any kind of mechanical or mercury pump.
Unexpected Influence of Size.
Suppose now a substance contains a great number of carbon molecules and a great number of water molecules, each of which has this residual affinity or power of clinging together well developed, what may be expected to be the result? Surely, the formation of a molecule consisting of thousands or hundreds of thousands of atoms, constituting substances more complex even than those already known to or analysable by organic chemistry; and if these complex molecules likewise possess the adhesive faculty, a grouping of millions or even billions of atoms may ultimately be formed. (A billion, that is a million millions, of atoms is truly an immense number, but the resulting aggregate is still excessively minute. A portion of substance consisting of a billion atoms is only barely visible with the highest power of a microscope; and a speck or granule, in order to be visible to the naked eye, like a grain of lycopodium-dust, must be a million times bigger still.) Such a grouping is likely to have properties differing not only in degree but in kind from the properties of simple substances.
For it must not be thought that aggregation only produces quantitative change and leaves quality unaltered. Fresh qualities altogether are liable to be introduced or to make their appearance at certain stages—certain critical stages—in the building up of a complex mass (cf. p. 71).
The habitability of a house, for instance, depends on its possessing a cavity of a certain size; there is a critical size of brick-aggregate which enables it to serve as a dwelling. Nothing much smaller than this would do at all. The aggregate retains this property, thus conferred upon it by size, however big it may be made after that; until it becomes a palace or a cathedral, when it may perhaps reach an upper limit of size at which it would be crushed by its own weight, or at which the span of roof is too great to be supported. But the difference, as regards habitability, between a palace and a hovel is far less than that between a hovel and one of the air-holes in a brick or loaf, or any other cavity too small to act as a human habitation. The difference as regards habitability is then an infinite difference.
To take a less trivial instance; a planet which is large enough to retain an atmosphere by its gravitative attraction differs utterly, in potentiality and importance, from the numerous lumps of matter scattered throughout space, which, though they may be as large as a haystack or a mountain or as the British Isles, or even Europe, are yet too small to hold any trace of air to their surface, and therefore cannot in any intelligible sense of the word be regarded as habitable. One of the lumps of matter in space can become a habitable planet only when it has attained a certain size, which conceivably it might do by falling together with others into a complex aggregate under the influence of gravitative attraction. The asteroids have not succeeded in doing this, but the planets have; and, accordingly, one of them, at any rate, has become a habitable world.